trans-2,5-Dialkylpyrrolidinyl-containing phosphinamines. Synthetic and mechanistic studies in Pd-catalysed asymmetric allylic alkylation

The preparation of new phosphinamine ligands possessing an enantiopure trans-2,5-dialkylpyrrolidinyl unit linked by a rigid o-phenylene bridge to a diphenylphosphine is described. Only that route forming the trans-2,5-dialkylpyrrolidine in the final step from (2-aminophenyl)diphenylphosphine proved successful. The cyclocondensation proceeded in 48% and 27% yields, respectively, for the dimethyl- and diethyl-analogues. Their palladium complexes were prepared and applied to the test of enantioselective alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate in high chemical yields but with moderate enantioselectivities of up to 34% ee.¹H NMR spectra of the η³-allyl Pd complexes of four trans-2,5-dialkylpyrrolidine-containing ligands were analysed in an attempt to explain the results obtained. In the cases of the 1,3-diphenylallyl complexes, two diastereomers were observed for all four ligands and their configurations were assigned with the aid of COSY and NOESY experiments. The catalytic results obtained are best interpreted by the reaction proceeding with nucleophilic attack on the allyl trans to the phosphorus donor atom of the major diastereomer.